In electronic circuits, current signal is usually taken as the input signal of the control circuitry and protection circuitry. Current signals can allow electric circuits to operate under stable and reliable circumstances. Therefore, in order to grasp the variations of current signals in electric circuits, an accurate current signal sensing technique is necessary to be built to reflect the actual electric currents. Current signals in typical electric circuits include switching current, input current, load current, and so on. Thus far, several current sensing techniques have been directly or indirectly applied to a variety of circuit topologies. FIG. 1 shows a current sensing circuit for sensing a load current and a power converter 100 incorporating such current sensing circuit according to the prior art. It should be noted that similar circuit elements are labeled with the same reference numerals throughout the specification. As shown in FIG. 1, the power converter 100 includes an input capacitor Cin connected to the input terminals of the power converter 100 for removing high-frequency noises from the input voltage Vin. The power converter 100 further includes switches Qa-Qd connected in parallel with the input capacitor Cin. The power converter 100 further includes a transformer T100 and a LLC resonant circuit consisted of a resonant inductor Lr, a resonant capacitor Cr, and a resonant inductor Lm, in which the resonant inductor Lm can be the magnetizing inductance of the transformer T100. The LLC resonant circuit (Lr, Cr, Lm) is configured to generate resonance to transfer the energy of the input voltage Vin to the primary side Np100 of the transformer T100 according to the switching operation of the switches Qa-Qd. The transformer T100 is configured to transfer the energy at its primary side Np100 to its secondary side Ns100 according to the switching operation of the switches Qa-Qd, thereby inducing an AC current across its secondary side Ns 100. The induced AC current is rectified by the synchronous rectifier (SR1,SR2) located at the secondary side Ns100 of the transformer T100 into a full-wave rectified DC current, thereby generating an output DC voltage Vo across the output capacitor Co to power a load RL. In addition, a current sensing resistor Rs is placed at the secondary side Ns100 of the transformer T100 and the output side of the synchronous rectifier (SR1, SR2) for sensing the load current Io of the power converter 100. The current flowing through the current sensing resistor Rs denotes the actual load current. The current sensing technique revealed in FIG. 1 is advantageous in terms of simple circuit structure and the steadiness of current sensing accuracy without being affected by the parasitic parameter by the converter 100. Nonetheless, the large load current flowing through the current sensing resistor Rs will cause considerable power loss, which in turn results in low power conversion efficiency. Besides, the heat generated as a result of the large load current flowing through the current sensing resistor Rs is difficult to dissipate. It should be noted that the output voltage Vo developed across the output capacitor Co is constant, and thus the average current flowing through the output capacitor Co is zero. Therefore, the average value of the secondary current Is of the power converter 100 is equal to the load current Io, i.e. the output current of the power converter 100. It should be noted that the switching control circuit which is used to control the switching of the switches Qa-Qd is not shown in the power converter 100 of FIG. 1 for simplicity.
FIG. 2 shows another kind of current sensing circuit for sensing a load current and a power converter 200 incorporating such current sensing circuit according to the prior art. In FIG. 2, a current transformer CT has a primary winding CTNP connected to the output capacitor Co and a secondary winding CTNS. The current transformer CT is placed between the secondary side Ns100 of the transformer T100 and the output capacitor Co, or placed between the output terminal of the synchronous rectifier (SR1, SR2) and the output capacitor Co. The load current Io flows through the primary winding CTNP of the current transformer CT and a proportional current is induced across the secondary winding CTNS of the current transformer CT. The proportional current is rectified by the diode rectifier DR200 connected to the secondary winding CTNS of the current transformer CT, thereby generating a current sensing signal by the current sensing resistor Rs connected to the diode rectifier DR200. However, it is well known the current transformer CT is a magnet element and the load current Io is relatively large. The large load current flowing through the primary winding CTNP of the current transformer CT will cause considerable coil loss, thereby lowering the power conversion efficiency. It should be noted that the switching control circuit which is used to control the switching of the switches Qa-Qd is not shown in the power converter 200 of FIG. 2 for simplicity.
FIG. 3 shows another kind of current sensing circuit for sensing a load current and a power converter 300 incorporating such current sensing circuit according to the prior art. In FIG. 3, a current transformer CT is placed at the primary side Np100 of the transformer T100, and has a primary winding CTNP connected between the resonant inductor Lm of the LLC resonant circuit (Lr, Cr, Lm) and the primary winding Np100 of the transformer T100 and a secondary winding CTNS. Because the primary current Ip is proportional to the secondary current Is of the power converter 300 and the proportion between the primary current Ip and the secondary current Is depends on the turn ratio of the transformer T100, the load current can be sensed by sensing the primary current Ip. In addition, a synchronous rectifier SR is connected to the secondary winding CTNS of the current transformer CT for synchronously rectifying the current induced across the secondary winding CTNS of the current transformer CT. Also, a current sensing resistor Rs is connected to the synchronous rectifier SR to generate a current sensing signal. It should be noted that the switching control circuit which is used to control the switching of the switches Qa-Qd is not shown in the power converter 300 of FIG. 3 for simplicity. If the power converter 300 is a buck converter, the primary current Ip is definitely lower than the secondary current Is. Therefore, the coil loss incurred by the current transformer CT will be greatly reduced. However, the current transformer CT is used for signal conversion only, and the synchronous rectifier SR is used to perform accurate synchronous rectification to the current sensing signal according to different load current waveforms. The gate driving signal of the synchronous rectifier SR is generated by the load current waveform, and thus the synchronous rectifier SR of FIG. 3 requires a set of complicated gate driving signal generating circuit as a driver. In this manner, the current sensing circuit of FIG. 3 will complicate the circuit design and lower the reliability, and further increase the manufacturing cost.
There is a tendency to develop a current sensing technique and a current sensing signal comparing technique for accurately sensing the current signal of a power converter and applying the result of sensing to regulate or protect the power converter. Or otherwise, the result of sensing can be reported to the power converter.